Passivhaus Affiliate

UK's first pre-certified step-by-step EnerPHit

A 1978 mid-terrace townhouse in London has become the first certified step-by-step EnerPHit retrofit in the UK. The 3-storey property is both home and project of architect Harry Paticas, of PHT member Arboreal Architecture.

4BL EnerPHit Retrofit (step by step) Front elevation ©Harry Paticas


Step-by-step EnerPHit is designed to facilitate long-term deep retrofit projects, by planning measures from the outset which will lead ultimately to achievement of all the usual EnerPHit targets, however many years the project may take to complete. Once the first step in the plan has been successfully accomplished, and demonstrable energy savings of a minimum of 20% (easily surpassed in this case) have been achieved, the whole project can be pre-certified.

4BL EnerPHit - Frosty roof ©Harry Paticas

On this project, the first step in the phased renovation was completed in May 2018. The loft has been insulated, work replacing windows, adding internal wall insulation, extensive airtightness works to the 2nd floor ceiling and front elevation has begun, and the MVHR system has been installed and commissioned. These measures have already led to a reduction in the space heat demand of 53%. And using monitoring equipment from Airvisual, Harry is able to show that CO2 levels are now down to 600ppm and the building has a constant RH of 50-55%. The effect of the loft insulation was already visible earlier in the year, when frost, evaporated from neighbouring roofs by the heat escaping from inside, clearly settled on the roof of number 4.


Key facts

  • Cost: £39,000 at 88sqm to EnerPHit step 2

  • Predicted energy demand: 22kWh/m².yr

  • Construction type: Brick and block, part cavity wall

  • Internal wall insulation: Cork

  • Loft insulation: Recycled polyester

  • Floor: XPS

The step-by-step certification route is especially useful when either it’s not practicable to carry out the whole retrofit at once or when all the necessary funds are not immediately available to do so. In this case only a very small budget was available to carry out building work after the property was purchased in 2016, and the house will be home for Harry and his family throughout the project.


One of the requirements of all Passivhaus certification is the provision of detailed drawings showing all the standard assemblies and connections of the building envelope. For this project - demonstrating how much art there can be in technical detail - each drawing (50 in total) is a hand-drawn watercolour executed with great care, even showing the end grain of the floor joists.

Detailed drawing 4BL 1/100 - Harry Paticas

Not only is Harry the architect and Passivhaus Designer on the project, but he is also carrying out most of the building work himself. The same care evident in his drawings can also be seen in work on site. The cork insulation will be exposed internally, so every cut must be neat as a pin and the finish smooth.


Throughout the project, as well as EnerPHit certification, Harry is also focusing on exploring the complexity and difficulty of step-by-step retrofit, on learning by doing as a way of informing best building practice, on specifying materials with low embodied carbon, on reducing construction waste, and on improving indoor air quality.

In future stages, the addition of more internal wall insulation, triple glazed windows, and finally floor insulation will bring energy demand savings down by in total 90%. The icing on top will be PV added at the final stage, to produce renewable energy at a rate of approx 50 kWh/m2.

 Steps to 90% savings

Step-by-step to 90% savings - step 2 already complete.


All images ⓒ Harry Paticas

Find out more

Do not miss the opportunity to visit the scheme on Saturday 10th November, as part of the International Passivhaus Open Days. Hear more about this whole house retrofit scheme (and additional low carbon retrofit solutions) at this year’s UK Passivhaus Conference – Passivhaus and the road to zero carbon – on 13 November in Leeds. Harry will be presenting the project’s measured performance and whole house approach alongside Wilmcote House, a UK example of adopting a similar staggered deep green retrofit on a large tower block

Passivhaus and the road to zero carbon

Further information

Passivhaus Retrofit


#4BL on Twitter 

Passivhaus and the road to zero caarbon 

2018 International Passivhaus Open Days

Previous PHT story: Progress being made at Wilmcote House EuroPHit project – 28 April 2015

20th October 2018

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